In the field of tissue engineering, it is well known that tissue constructs can be produced which are implantable in the human (or animal) body, to aid in the repair of damaged tissue. The tissue in question may, for example, have experienced trauma or be the subject of a degenerative condition. The tissue constructs comprise scaffolds which have typically been seeded with cells taken from a patient (autogenous cells), or from a suitable donor (allogenic cells), and are implanted in the body in the region of the damaged tissue. The construct may replicate at least part of the function of the damaged tissue, and/or may promote the growth of natural tissue in the damaged region.
A particular focus of recent research has concerned attempts to form tissue engineered constructs for cartilage repair, which can be implanted in the body of a patient who has experienced damage to, or degeneration of, natural cartilage tissue. Examples of the former would be patients who have experienced cartilage injury associated with common joint injuries such as to the anterior cruciate ligament (ACL).
Cartilage damage can eventually lead to osteoarthritis, causing pain and reduced joint mobility, seriously compromising the affected individual's quality of life. It is well recognised that cartilage has a poor capacity for spontaneous self-repair, in part because of its low cellularity and the lack of vascular and lymphatic systems necessary for efficient healing. In addition, any neo-tissue that is deposited is likely to be destroyed by the stresses acting within joints during daily activities, because it is mechanically weak. Intervention is required to maintain quality of life. However, so far, surgical treatments for articular cartilage defects have not been consistently effective in preventing the recurrence of damage.
One potential strategy for repairing cartilage is the implantation of in-vitro produced cartilage constructs that have similar matrix composition, and mechanical properties, to those of the surrounding native cartilage. The principles of tissue engineering would be followed in creating such cartilage constructs, where appropriate mechanical stimulation would be applied onto cells seeded onto a suitable 3D scaffold to promote a chondrocyte-like phenotype and matrix. It is well documented that biochemical stimulation is a pre-requisite for successful differentiation/re-differentiation of cells and desired tissue deposition. Mechanical stimulation is also a highly influential factor in the formation of tissue, especially musculoskeletal tissue.
In order for mechanical loading to elicit a desired mechanotransductive effect on a construct, it is considered that the following criteria should be met: 1) the scaffold should be sufficiently compliant and populated by viable cells with a chondrocyte-like phenotype and initially surrounded by sufficient extracellular matrix (ECM) so that, when loaded, the scaffold will deform and the applied force will be transmitted through the matrix to the cells via appropriate integrins, leading to further deposition and remodelling of the ECM resulting in a cartilage-like construct; and 2) to achieve the above, the values of compressive strain applied on to the constructs should be within the physiological range, which maintain the native tissue's functional properties and therefore likely to induce the desired anabolic effects.
However, to date, the application of compressive loading has had limited success in creating mechanically functional constructs that are suitable to be implanted. It has been suggested that, because these constructs had moduli that were significantly lower than those of the surrounding native cartilage, they would deform under joint loading by a greater amount post-implantation, generating excessive shear at the construct-tissue interface, impeding integration. In addition, such constructs have been considered unable to withstand the combination of high compressive and shear stresses arising in the joint in question.
One prior technique involving the application of pulsatile hydrostatic pressure (PHP) is discussed in the paper by Luo and Seedhom entitled “Light and low-frequency pulsative hydrostatic pressure enhances extracellular matrix formation by bone marrow mesenchymal cells: An in-vitro study with special reference to cartilage repair”, published in the Proceedings of the Institution of Mechanical Engineers, Vol. 221, Part H: Journal of Engineering in Medicine, May 2007. The technique involves forming circular pads of non-woven filamentous material, subjecting the pads to plasma treatment to confer hydrophilicity, and sterilising the pads with gamma irradiation, to form scaffolds. Ovine bone marrow cells are then seeded on to the scaffolds, and the resulting cell-scaffold constructs are cultured in a chondrogenic medium. The cell-scaffold constructs are then subjected to (PHP) of a fixed, low magnitude of 0.1 MPa, and with a sinusoidal wave pattern of a fixed, low frequency of 0.25 Hz, for 30 minutes every day for 10 days. This stimulates tissue matrix deposition in the cell-scaffold construct. Biochemical assays demonstrated that DNA content in the group of PHP constructs subjected to this technique was 1.5 times greater than in a control group at day 10. However, it has been found that the tissue matrix formed does not provide the construct with sufficient modulus, compared to that of natural cartilage tissue, so that the scaffold will not have sufficient stiffness to form an effective implant.
Another prior technique involving the application of compressive loading is disclosed in the paper by Mauck et al entitled “Functional tissue engineering of articular cartilage through dynamic loading of chondrocyte-seeded agarose gels”, published in the Journal of Biomechanical Engineering, 252/Vol. 122, June 2000. The technique involves forming constructs by seeding hydrogel discs with cells derived from harvested bovine articular chondrocytes, and loading the cell-seeded construct discs using a bioreactor driven by a motor via a cam arrangement and monitoring the load applied with a load cell. The discs are loaded dynamically in unconfined compression with a peak-to-peak compressive strain of 10%, for an hour at a frequency of 1 Hz three times per day, five days per week, over a four week period. This stimulates tissue matrix deposition in the constructs. Whilst results demonstrated that such dynamically loaded constructs yielded an increase in equilibrium aggregate modulus over free swelling controls after 28 days of loading, it has again been found that the tissue matrix formed does not provide the constructs with sufficient modulus to form an effective implant.
If constructs with mechanical properties comparable to those of the native cartilage could be implanted, they would be more likely to survive the rigors of the mechanical environment within the joint in question, to integrate with the surrounding native cartilage, and to produce long term repair. However, to date, none of the techniques employed have produced cartilage constructs having an acceptable elastic modulus (stiffness) and/or yield strength to adequately replicate native cartilage tissue.
The problems discussed above are not restricted to cartilage tissue, and so apply equally to other body tissues.
It is an object of the present invention to obviate or mitigate at least one of the foregoing disadvantages.